Alpha Carbon Chemistry: Enols and Enolates

Dieckmann condensation – An Intramolecular Claisen Reaction

Just like the aldol reaction, intramolecular Claisen reactions occur when five- or six-membered rings can be formed. These are called the Dieckmann condensation where one ester, of the same molecule, serves as an electrophile and the other is deprotonated and acts as a nucleophile. Here is the mechanism for a Dieckmann reaction forming a six-membered ring:

 

 

The product is a cyclic β-keto ester which has a very acidic alpha hydrogen. And it is this proton that makes any Claisen reaction possible since it is removed in an irreversible acid-base reaction which is the driving force that shifts the equilibrium in favor of the condensation.

This and the details for choosing a proper base for a Claisen and Dieckmann condensation are covered in this post.

 

Regiochemistry of Dieckmann condensation

Now, the example we discussed above is a symmetrical molecule and it wouldn’t matter which of the ɑ hydrogens is deprotonated. However, when this is not the case and there are two different ɑ positions in the molecule, a strong base can be used for selective enolate formation just like in the crossed Claisen and Aldol reactions:

 

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